In this application, a fuse isn't really necessary, since you have the upstream breaker at 15A or 20A. Obviously you would need to bear in mind you wouldn't really want a continuous load of more than 10A on it. The relays apparantely say 15A on them, but that doesn't necessarily mean the screw connectors and solder traces on the board can handle that much. You could certainly put in a fuse if you want to. Some people like to have a 1A fuse going to the STC's control power. I would imagine it has a fusible link on the circuit board anyway for the control power though I could be wrong.